The FBI improperly and, in some cases, illegally used the USA Patriot Act to secretly obtain personal information about people in the US, a justice department audit found today.

According to the highly critical 126-page report, FBI agents sometimes demanded the information without proper authorisation and at times improperly obtained telephone records in circumstances that were not emergencies.

It blamed agent error and shoddy record-keeping for most of the problems - and did not find any indication of criminal misconduct - but has confirmed the worst fears of civil liberties groups on the use of the so-called national security letters.

A power outlined in the post-9/11 Patriot Act, the letters are a form of administrative subpoena used in suspected terrorism and espionage cases to obtain thousands of telephone, business and financial records without prior judicial approval.

"We believe the improper or illegal uses we found involve serious misuses of national security letter authorities," the audit concluded.

The audit found that the number of national security letters issued by the FBI soared after the Patriot Act became law. In 2000, the FBI issued an estimated 8,500 letters.

By 2003, however, that number had jumped to 39,000. It rose again the next year, to about 56,000 letters in 2004, and dropped to approximately 47,000 in 2005.

Over the entire three-year period, the audit found the FBI had issued 143,074 national security letters requesting customer data from businesses.

The FBI director, Robert Mueller, called the audit "a fair and objective review of the FBI's use of a proven and useful investigative tool".

The finding "of deficiencies in our processes is unacceptable", Mr Mueller said in a statement.

"We strive to exercise our authorities consistent with the privacy protections and civil liberties that we are sworn to uphold. Anything less will not be tolerated.

"While we've already taken some steps to address these shortcomings, I am ordering additional corrective measures to be taken immediately."

The use of national security letters since the September 2001 attacks has been highly controversial. They were once used only in espionage and terrorism cases, and then only against people suspected as agents of a foreign power.

With the passage of the Patriot Act, their use rocketed and was allowed against Americans during any investigation. The law also allowed other agencies such as the homeland security department to issue the letters.

The greatest concern for civil liberties groups is that unlike search warrants, security letters are issued without prior judicial approval and require only the approval of the agent in charge of a local FBI office. A supreme court ruling in 2004 forced revisions of the Patriot Act to permit greater judicial review, without requiring advance authorisation.

Today's audit means further trouble for the attorney general, Alberto Gonzales, who has already infuriated Congress by abruptly dismissing eight government attorneys without explanation.